A medical implant can replace, support, or act as a missing biological structure. Examples of medical implants include orthopedic implants, bioscaffolding, and endoprostheses such as stents, covered stents, stent-grafts, bone screws, and aneurysm coils. A medical implant can also add a new function to the body. For example, medical implants can include identification tags, communication devices, and/or pacemaking electrodes.
Endoprostheses can be implanted in various body passageways such as arteries, other blood vessels, and other body lumens (e.g., neural pathways). These passageways sometimes become occluded or weakened. For example, the passageways can be occluded by a tumor, restricted by plaque, or weakened by an aneurysm. When this occurs, the passageway can be reopened or reinforced, or even replaced, with an endoprosthesis. An endoprosthesis is typically a tubular member placed in a lumen in the body.
Endoprostheses can be delivered inside the body by a catheter. The catheter supports the endoprosthesis in a compacted or reduced-size form as the endoprosthesis is transported to a desired site. Upon reaching the site, the endoprosthesis is expanded, for example, so that it can contact the walls of the lumen.
The expansion mechanism can include forcing the endoprosthesis to expand radially. For example, the expansion mechanism can include the catheter carrying a balloon, which carries a balloon-expandable endoprosthesis. The balloon can be inflated to deform and to fix the expanded endoprosthesis at a predetermined position in contact with the lumen wall. The balloon can then be deflated and the catheter withdrawn.
In another delivery technique, the endoprosthesis is formed from an elastic material that can be reversibly compacted and expanded, e.g., elastically or through a material phase transition. During introduction into the body, the endoprosthesis is restrained in a compacted condition. Upon reaching the desired implantation site, the restraint is removed, for example, by retracting a restraining device such as an outer sheath, enabling the endoprosthesis to self-expand by its own internal elastic restoring force.
Endoprostheses can sometimes carry a drug, such as an antiproliferative, to reduce the likelihood of restenosis, i.e., reclosure of the vessel due to immune reactions by the body at the treatment site. For example, a drug-eluting layer can be coated onto an endoprosthesis.